I would try something like this:

Include "modedefs.bas" ' Include serial modes

SO var portb.0 ' Define serial out pin <-------You have to define a port
SI var portb.1 ' Define serial in pin <--------You have to define a port
B0 var byte

loop:
Serin SI,396,[B0]' B0 = input character
If (B0 < "a") or (B0 > "z") Then ' If lower case, convert to upper
B0 = B0 - $20
endif
Serout SO,396,[B0] ' Send character <------You don't need print command
Goto loop ' Forever


Try this and see what happens.

Chris